diff --git a/poetry.lock b/poetry.lock
index 7b9fbd68..a250de86 100644
--- a/poetry.lock
+++ b/poetry.lock
@@ -1137,13 +1137,13 @@ docs = ["sphinx (>=8,<9)", "sphinx-autobuild"]
 
 [[package]]
 name = "mypy-boto3-s3"
-version = "1.35.76.post1"
-description = "Type annotations for boto3 S3 1.35.76 service generated with mypy-boto3-builder 8.6.3"
+version = "1.35.81"
+description = "Type annotations for boto3 S3 1.35.81 service generated with mypy-boto3-builder 8.6.4"
 optional = false
 python-versions = ">=3.8"
 files = [
-    {file = "mypy_boto3_s3-1.35.76.post1-py3-none-any.whl", hash = "sha256:fd4a8734c3bb5a2da52e22258b1836a14aa3460816df25c831790e464334021f"},
-    {file = "mypy_boto3_s3-1.35.76.post1.tar.gz", hash = "sha256:34ac4cacf8acdafa6e71a2810116b2546376f241761f9eec6ac5a9887309372b"},
+    {file = "mypy_boto3_s3-1.35.81-py3-none-any.whl", hash = "sha256:6af1d815ff2cc8e32ca1190c7387f94341c1607444f958ac283aa10b1d11db08"},
+    {file = "mypy_boto3_s3-1.35.81.tar.gz", hash = "sha256:fe1a6860c0ca7016e24089819433c0d5835d4a57635bb42628645b71271b946c"},
 ]
 
 [package.dependencies]
diff --git a/src/caselawclient/models/identifiers/unpacker.py b/src/caselawclient/models/identifiers/unpacker.py
index c98937a8..7233db86 100644
--- a/src/caselawclient/models/identifiers/unpacker.py
+++ b/src/caselawclient/models/identifiers/unpacker.py
@@ -1,4 +1,5 @@
 from typing import Optional
+from warnings import warn
 
 from lxml import etree
 
@@ -19,12 +20,13 @@ def unpack_all_identifiers_from_etree(identifiers_etree: Optional[etree._Element
         return identifiers
     for identifier_etree in identifiers_etree.findall("identifier"):
         identifier = unpack_an_identifier_from_etree(identifier_etree)
-        identifiers.add(identifier)
+        if identifier:
+            identifiers.add(identifier)
     return identifiers
 
 
-def unpack_an_identifier_from_etree(identifier_xml: etree._Element) -> Identifier:
-    """Given an etree representation of a single identifier, unpack it into an appropriate instance of an Identifier."""
+def unpack_an_identifier_from_etree(identifier_xml: etree._Element) -> Optional[Identifier]:
+    """Given an etree representation of a single identifier, unpack it into an appropriate instance of an Identifier if the type is known (otherwise return `None`)."""
 
     namespace_element = identifier_xml.find("namespace")
 
@@ -33,6 +35,11 @@ def unpack_an_identifier_from_etree(identifier_xml: etree._Element) -> Identifie
             "Identifer XML representation is not valid: namespace not present or empty"
         )
 
+    # If the identifier namespace isn't known, fail out
+    if namespace_element.text not in IDENTIFIER_NAMESPACE_MAP:
+        warn(f"Identifier type {namespace_element.text} is not known.")
+        return None
+
     kwargs: dict[str, str] = {}
 
     for attribute in IDENTIFIER_UNPACKABLE_ATTRIBUTES:
diff --git a/tests/models/identifiers/test_identifer_unpacking.py b/tests/models/identifiers/test_identifer_unpacking.py
index 0911d37e..499d9bbd 100644
--- a/tests/models/identifiers/test_identifer_unpacking.py
+++ b/tests/models/identifiers/test_identifer_unpacking.py
@@ -1,3 +1,4 @@
+import unittest
 from unittest.mock import patch
 
 from lxml import etree
@@ -7,7 +8,7 @@
 from caselawclient.models.identifiers.unpacker import unpack_all_identifiers_from_etree, unpack_an_identifier_from_etree
 
 
-class TestIdentifierUnpacking:
+class TestIdentifierUnpacking(unittest.TestCase):
     @patch("caselawclient.models.identifiers.unpacker.IDENTIFIER_NAMESPACE_MAP", {"test": TestIdentifier})
     def test_unpack_identifier(self):
         xml_tree = etree.fromstring("""
@@ -24,6 +25,21 @@ def test_unpack_identifier(self):
         assert unpacked_identifier.uuid == "2d80bf1d-e3ea-452f-965c-041f4399f2dd"
         assert unpacked_identifier.value == "TEST-123"
 
+    @patch("caselawclient.models.identifiers.unpacker.IDENTIFIER_NAMESPACE_MAP", {"test": TestIdentifier})
+    def test_unpack_unknown_identifier(self):
+        xml_tree = etree.fromstring("""
+            <identifier>
+                <namespace>unknown</namespace>
+                <uuid>2d80bf1d-e3ea-452f-965c-041f4399f2dd</uuid>
+                <value>UK-123</value>
+            </identifier>
+        """)
+
+        with self.assertWarnsRegex(Warning, "Identifier type unknown is not known."):
+            unpacked_identifier = unpack_an_identifier_from_etree(xml_tree)
+
+        assert unpacked_identifier is None
+
 
 class TestIdentifierPackUnpackRoundTrip:
     @patch("caselawclient.models.identifiers.unpacker.IDENTIFIER_NAMESPACE_MAP", {"test": TestIdentifier})